Heterodyne eliminator communication system



Dec. 26, 1950 J. L. A, MGLAUGHLIN HETERODYNE ELIMINATOR COMMUNICATIONSYSTEM Filed July 26, 1949 4 Sheets-Sheet 1 WWW Www

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Dec. 26, 1950 J. 1 A. MGLAUGHLIN HETERODYNE ELIMINATOR COMMUNICATILONSYSTEM Filed July 26, 1949 4 Sheets-Sheet 5 @Y www1 Dec. 26, 1950 J. L.A. MGLAUGHLIN HETERODYNE ELIMINATOR COMMUNICATION SYSTEM 4 Sheets-Sheet4 Filed July 26, 1949 2. ix d Patented Dec. 26, 1950 UNITED STATESPATENT OFFICE HETERODYNE ELIMINATOR COMMUNICA- TION SYSTEM 18 Claims.

This invention relates to a heterodyne elim inator communication system,and more particularly to such a system having a plurality of receivingsignal channels each passing a difierent group of wave segments spacedfrom each other in time and each channel passing frequencies comprisingonly a portion of the in telligence bearing waves.

One feature of the invention is that it provides an improved heterodyneeliminator communication system; another feature of the invention isthat it provides means for eliminating an undesired signal closelyadjacent a desired signal by providing receiving means wherein theintelligence bearing frequencies received at any given instant comprisea band narrower than the total band of intelligence bearing frequencies;a further feature of the invention is that it provides means foreliminating an undesired signal closely adjacent a desired signal bypro. viding a plurality of signal channels each passing a diierent groupof Wave segments spaced from each other in time, and each channelpassfeature of the invention is that it provides means for eliminatingundesired signals lying adjacent the center frequency of theintelligence bearing frequency band; yet a further feature of the'invention is that Ait provides means for cutting off the output of eachsignal channel when the output of the signal channel is below a prede--Atermined level; still an additional feature `of the invention is that itprovides means for eliminating undesired signals included in a singlesideband transmitted wave; and still a further feature of the inventionis that it provides improved means for eliminating undesired signalsclosely adjacent a desired signal which is in the form of a sequentiallymodulated wave.

Further features `and advantages of Vthe invention will be apparentfrom-the following de scription and from the drawings, in which:

Fig. l is a block diagram of one form of the invention;

Fig. 2 is a more detailed block diagram of a portion of the apparatus ofFig. 1;

Figs. 2A and 2B are frequency response graphs of portions of theapparatus shown in Fig. 2; Figs. 2A and 2B being located in the drawingsadjacent the apparatus they illustrate;

Figs. 2C and 2D are frequency response graphs of other portions of theapparatus of Fig. 2, Figs. 2C and 2D being located adjacent theapparatus they illustrate;

Fig. 3 is a combined graph of Figs. 2C and 2D;

Fig. .4 is a diagrammatic showing of the transmitted wave of theapparatus of Fig. 1;

Fig. 5 is a block diagram of a portion of a modified form of theinvention;

Fig. 6 is a combined frequency response graph of portions of theapparatus of Fig. 5;

Fig. '7 is a graph similar to Figure 6 including a diagrammatic showingof the operation of the apparatus;

Fig. 8 is a combined frequency response graph of another portion of theapparatus of Fig. 5; and

Fig. 9 is a diagrammatic showing of the transmitted wave for theapparatus of Fig. 5.

A radio signal normally comprises one or more bands of intelligencebearing waves, which may be associated with a carrier wave of fixedfrequency. The modulation of the carrier wave with the intelligencefrequencies it is desired to transmit; and this modulation may beaccomplished by varying the amplitud: of the carrie wave at thefrequencies oi the intelligence to be transmitted, or by varying thefrequency or phase of the carrier wave at a rate proportional to theintelligence freqlencies to be transmitted. Normally the carrier wavehas a sociatcd therewith two sdelfands, the sideband higher in frequencythan tl e carrier wave being known as a superior frequencies closlyadiacent the frequency of the desired signal which has been transmittedcannot be eliminated merely by making the receiv eceiving apparatus canbe designed with almost any desired amount of selectivity, but makingthe circuit too` selective results in chopping oi the outer edges',

ing apparatus more selective.

intelligence band results from` assegna of the sidebands and therebylosing part of the intelligence. Accordingly, the usual radio receiveris constructed to have a signal channel with a selectivity such that theentire sideband is received where single sideband transmission iscontemplated, and such that both upper and lower sidebands are receivedwhen double sideband transmission is contemplated.

When an undesired signal carrier wave lies within a sideband theordinary radio receiver system is unable to eliminate it and itheterodynes with the desired signal carrier to produce an audio wavewhich substantially or entirely drowns out the desired intelligence. Ihave heretofore discovered that selection of one or the other sidebandmay be made at the receiver to eliminate such an undesired wave, thesideband including the undesired wave being suppressed. In my patententitled, Heterodyne Eliminator, which issued on December 12, 1944 asNo. 2,364,863, I disclosed means for eliminating such an undesiredsignal by providing two separate waves at opposite sides of the carrierWave of the desired signal, and means for selectively causing one of theother of the waves to heterodyne with the desired signal to select oneof the sidebands. In my cao-pending application en titled, AutomaticI-I'eterodyne Eliminator, led August 5, 1946 as Serial No. 688,403,which issued September 6, 1949 as Patent No. 2,480,870, I disclosedmeans for eliminating an undesired signal by apparatus automaticallyoperable as a function Iof undesired signal conditions in the si'debandsfor selecting one of the waves to heterodyne with the desired signal;and in my I.

co-pending application entitled, Single Side- Iband Radio Equipment nledFebruary 28, 1948 as Serial No. 11,925, I disclosed apparatus whereinswitching from one sideband to the other is accomplished as a functionof the operation of the radio tuning means.

Reference may be had to the above patents for a full disclosure ofcertain of the apparatus shown in block form in this application, andwhile manual switching is shown in this application it will beunderstood that automatic switching means as disclosed in the abovePatent No. 2,480,870 or application No. 11,925 may be employed ifdesii-ed.

The above mentioned patent and co-pending applications depend for theiroperation upon existence of two sidebands positioned on'either side of acarrier wave. Recently, work has been done in developing radio apparatuswherein the band width is narrower than the spectrum of intelligence tobe transmitted. For example, certain systems have been developedutilizing a pulse-time or sequence modulated wave wherein a plurality ofmessages or other intelligence share the time on a single frequency or aband of frequencies. Such systems reduce the likelihood of theoccurrence of an undesired signal closely adjacent the desired signalbecause the frequencies utilized are less than the actual intelligenceband. However, reducing the band width of the transmitted signal willnot of itself necessarily reduce interference from undesired signals. Aspointed out above it reduces the probability that the undesired signalwill be present, but should an undesired signal vlie within the bandwidth that is actually transmitted, there has been no simple way ofeliminating the undesired signal. In other words, greater signalseparating ability (actual selectivity) can be had from a communicationsystem wherein the band width transmitted is greater than that of theintelligence spectrum than can be had from a system in which the bandwidth is equal to or less than the spectrum of intelligence frequencies.That the reduction of band width alone will not provide the eliminationof undesired signals may be seen from a consideration of radiotelegraphy or C. W. telegraphy which provides a system of communicationutilizing only a single frequency instead of a band of frequencies. Withthis system many messages may betransmitted by time sharing means usingonly a single frequency. However, it has been found that seriousheterodyne interference is present in most C. W. bands due to theexistence of undesired signals of the same frequency or very close in.frequency to the transmitted signal.

The present invention provides a means whereby an undesired signal maybe eliminated from a single sideband transmitted wave, and sequencemodulation, or time sharing, may be utilized in the system if desired.

Referring now more particularly to the drawings, in Figure 1 atransmitter I0 provides a wave shown diagrammatically in Figure 4. Thiswave comprises a single band of waves bearing intelligence to becommunicated and a plurality of carrier waves provided in alternatingsequence respectively at opposite sides of the band to provide aplurality of groups of segments, the segments of `each group recurringcyclically in timed sequence. Such a transmitter may be similar to theLE-system described in the article by F. A. Polkinghorn entitledCommercial Single Sideband Radiotelephone Systems, published in theDecember 1948 issue of Communication A more complete description of atransmitter of this type may be found in an article by A. A. Gswaldentitled A Short-Wave Single-Sideband Radiotelephone System, appearingin the December 1938 vissue of "Proceedings of the Institute of RadioEngineers.

In Figure 4 the band of waves bearing intelligence to be communicated isshown at I I, the entire band being stippled in the drawing. Carrierwaves are provided in alternating sequence respectively adjacent theupper and lower edge-s of the band to provide first and second lgroupsof segments. The carrier waves I2-a and IED adjacent the lower edges ofthe band provide a rst group of segments recurring alternately andcyclically in timed sequence, each segment including the carrier and the4sideband associated with the carrier, in this case being a superiorside band. Carrier waves I`3a and ISb are exemplary of a second group ofsegments including the carrier I3 and its inferior sideband.. l

While the frequencies utilized in the apparatus are not critical, but onthe contrary are subject to conversion to different frequencies `inportions of the radio receiver comprising a portion of the apparatus, itWill herein be assumed that the sideband II comprises a band of wavesbearing intelligence to be communicated and having a width of 4,000cycles. For example, the carrier I2 may have a frequency of 998 kc. andthe carrier I3 may have a frequency of 1,002 kc., so that the desiredsignal is symmetrically disposed about a center frequencyof 1000 kc.,which center frequency is designated by the broken line I4. Figure 4 anundesired carrier wave .I5 is shown as being vcontained within thesideband, vthis undesired wave being assumed to have a frequency of 999kc. so that it is spaced 1,000 cycles from izle carrier I2 and 3,000cycles from the carrier InY The desired signal is transmitted from anantenna lila of the transmitter I, and may be received by a receiverdesignated generally at I6 comprising an antenna II coupled to anamplifier and frequency converter section I8. The amplifier andfrequency converter section may comprise the first stages of aconventional superheterodyne receiver, including one or more stages ofradio frequency amplification, a local oscillator and mixer forconverting the transmitted frequency of the desired signal to a lowerfrequency of the desired signal commonly known as an intermediatefrequency, and may include one or more stages of intermediate frequencyamplification. The amplifier and frequency converter output is coupledto a first signal channel including a first filter-amplifier coupled toa detector 2I for demodulating the received carrier. An audio filter 22is coupled to the detector, and a squelch circuit 23 is coupled to theoutput of the audio filter. A portion of the output of the amplifier andfrequency converter I3 is passed through a second signal channelincluding a second filter-amplifier 2li, a second demoduator or detector25, an audio filter Z and a squelch circuit 21. Both squelch circuits 23and 2T are coupled to an audio frequency amplifier 28, which in turn iscoupled to a speaker 29.

Figure 2 shows a portion of the receiver` in greater detail and will beused to illustrate the operation of the system. The last stage ofintermediate frequency amplification is shown at 18a, this stagecomprising a portion of the amplifier and frequency converter I8. Theintermediate frequency amplifier Ia is shown in Figure 2 as beingcoupled to a mixer stage 30, rather than as being coupled inconventional fashion directly to the filter-amplifiers 20 and 2li, theinput of the mixer being also connected to the movable termlnal 3Ia of aswitch designated generally at 3l.

One of the stationary terminals 3Ib of this switch is connected to anoscillator A and the other stationary terminal 3 I e of the switch isconnected to another oscillator 13. These oscillators are similar tothose shown in my Patent No. 2,364,863

above referred to and in my co-pending application Serial No. 688 403above referred to, and reference may be had to said patent andapplication for a complete disclosure thereof. rlhe intermediatefrequency amplifier Ita and mixer 30 are 0f conventional constructionWell known to the art and will not be described in detail here.

The filter-amplifiers 20 and 24 are conventional intermediate frequencyamplifiers having selectivity response curves as shown respectively inFigures 2A and 2B and will not be described in detail. Similarly thedetectors 2l and 25 are conventional; the filters 22 and 23 areconventional audio-frequency lters having response curves as shown inFigures 2C and 2D; and the audiofrequency amplifier 28 and the speaker29 are of conventional construction. For a disclosure of the squelchcircuits 23 and 21, reference may be had to pages 185 and 186 of TheElectronic Engineering Handbook by Batcher and Moulic. The receiverillustrated may be assumed to operate with an intermediate frequency of500 kc. although this frequency again is merely exemplary and is notcritical. It is assumed that the signal of Figure 4 has been convertedin the earlier stages of the receiver so that in Figure 2 the desiredsignal (the band II of Fig. 4) has a center frequency of 500 kc.Oscillator A may have a frequency of 600 kc. (being a frequency equal tothe mean frequency of the desired signal plus kc.) and oscillator B mayhave a frequency of 400 kc. (being a frequency equal to the meanfrequency of the desired signal minus 100 kc.) and one or the other ofthese oscillators is always connected to the input of the mixer 30 sothat the oscillator wave heterodynes with the desired signal from theamplifier IBa to provide a converted desired signal having a meanfrequency of 100 kc. This converted desired signal is effectively splitin half at the mean frequency and each half (or at least a portionthereof) is passed through a different one of the two signal channels sothat the carrier I2 and its side-band are demodulated in one channel andthe carrier I3 Vand its sideband are demodulated in the other channel.It should be noted that when the sideband is split into two portionscentered at the mean frequency, only one-half of the sideband contains afull spectrum of information. Referring for a moment to Figure 4, theupper half (in the drawing) of the sideband segments associated with thecarrier segments I2 bears information frequencies from 2,000 cycles to4,000 cycles. Similarly the upper half of the sideband segmentsassociated with the carrier segments I3 bears intelligence frequenciesfrom 0 to 2,000A

cycles. Inasmuch as the carrier is cyclically shifted in frequencybetween the lower and upper edges of the sideband it will be seen thatonly half of the sideband (as shown by the shaded area I6' of Figure 4)contains a full spectrum of intelligence from 0 to 4,000 cycles, eachhalf of this spectrum sharing the band in time with the other half ofthe spectrum.

Assuming that oscillator BI is coupled to the mixer` 30 as shown,carrier I2 will be hetercdyned from its I. F. frequency of 498 kc. to afrequency of 98 kc. and carrier I3 will be heterodyned from itsintermediate frequency of 502 kc. to a converted frequency of 102 kc.However, the undesired signal I 5 will be converted from its I. F.frequency of 499 kc. to a converted frequency of 99 kc. Theserelationships are shown in Figure 2A, wherein the frequency response oracceptance pattern of the lter-arnplifier 20 is shown by the curve 20aand wherein the converted center frequency (100 kc. in the examplegiven) is indicated on the graph at zero. On the graph of Figure 2A theconverted carrier is shown at I2' as having a frequency of 0 minus 2(100 kc. minus 2 kc.), this converted carrier being within the band passof the filter amplifier 20. The other converted carrier is suppressed,this converted carrier being shown at I3' in broken lines as being t,outside the band pass of the filter-amplifier.

However, the converted undesired signal I5' being only one kilocyclebelow the center frequency, is also within the band pass of thefilter-amplifier.

The received desired signal is demodulated in the detector 2| inconventional manner, and after demodulation the audio or intelligencebearing Waves are coupled into the filter 22. As indicated in Figure 2,this filter is a low pass filter, having a pass range from zero to 2,000cycles as indicated graphically in'Figure 2C. Thus, in the examplegiven, not only the intelligence bearing waves, but also the undesiredsignal I5 would pass through the audio filter, and the heterodyning ofthis undesired signal with the converted desired signal I2' wouldproduce an undesired beat note of 1,000 cycles which would pass throughthe low pass filter and through the squelch circuit and the audiofrequency amplifier and would produce a 1000 cycle tone in the speaker29.

" lorder to V'avoid this undesirable .action the frequency of thesegments received is inverted to invert the frequency of a portion ofthe band passed by the signal channel so that the undesired signal iseliminated. This is accomplished by moving the switch Vmember 3m to theposition other than that shown where it connects the A oscillator withYthe mixer 30. Now the carrier I2 is converted from its intermediatefrequency of 49.8 kc. to a frequency of 102 kc.; the carrier I3 isconverted from its intermediate frequency of 502 kc. to a frequency of98 kc.; and the undesired signal is converted from its intermediatefrequency of 499 kc. to a frequency of 101 kc. In Figure 2A the desiredsignal carriers I2' and I3' now would be transposed from thepositionjshown, thek carrier I3 now appearing 2 kc. below the centerfrequency and within the band pass of the filter-amplifier, and thecarrier I2 now appearing 2`kc. -above the center frequency and outsidethe `pass range of the filter-amplifier so that the carrier I2 now issuppressed. The converted undesired signal is shown at I5" as having afrequency 1 kc. above the center frequency and as being outside the passrange of the filter amplifier 2D.

With the A oscillator switched in the received carrier i3 would be`demodulated in detector 2| in conventional manner, and the audio inteligence bearing Waves coupled to the audio low p..ss filter 22 so thatonly those audio frequencies in the range from to approximately 2 kc.would pass through the audio lter. While the converted undesired signali5 is outside the pass range of thelter-amplifier 29 `and is eliminatedby suchamplifier in the example given, if this converted undesiredsignal were closer to the center frequency, as for example if theconverted undesired signal had a frequency of 1.00.2 kc., the undesiredsignal still would be eliminated by the low pass filter. In this eventthe converted undesired signal would heterodyne With the desired signali3 to produce a beat note of 22.00 cycles and would be outside the passrange of the audio filter 2.2. AS .appears from the above discussion theshaded portion of only the segments associated with and including thecarrier wave i?. (in Fig. 4) passes through the signall channelcomprising theelements 210, 2| 22 and 23, and are fed from this signalchannel to the audio frequency amplifier and speaker 29;

The other group of segments, comprising those segments of the frequencyband associated with and including the carrier l2 pass through the sig'-nal channel comprising the elements 24, 25, 26 and 21 and are fed to theamplifier 28 and speaker 29. With the A oscillator connected to themixer as above described in order to eliminate the undesired signal, thecarrier l2 will be converted from its lintern-iediate frequency of 498vkc. .to a frequency of 102 kc.; the carrier I3 will be .converted fromits intermediate frequency of 502 kc. to a frequency of 98 kc.; and theundesired signal will be converted from its intermediate fre quency of499 kc. to a. .frequency rof 1.01 kc. Referring to Figure 2B thefrequency response curve of the ter-arnplifler .2li is designated at24a, this amplifier having a band pass from about 2 kc. below thevcenter frequency of 100 kc. to a frequencyof about .3 kc.. above :saidcenter frequency. I3 designates the converted carrier t3, and thiscarrier is outside thegpass range of the lteramplifier 24 andissuppressed. l2 designates the converted carrier i2, :and this carrier is:Within the pass range of the amplier and is received by the amplifier.'I5' designates the'converted undesired signal which is also w.thin thepass range of the amplifier. I5 shows the frequency posi tion which theundesiredv signal Would occupy if B oscillator were connected to themixer.

The received carrier I2 is demodulated in the` conventional manner inthe detector 25, this carrier of 102 kc. heterodyning with the undesiredWave I5' to produce a beat note of 1,000 cyc es However, this beat noteis eliminated in the high pass audio filter 25 the frequency responsecurve of which is shown at 26a in Fig. 2D. Since the filter 2S passes aband from 2 kc. to 4 kc. the shaded portion I5 (Fig. 4) of the group ofsegments associated lwith the carrier I2 will Ypass through this filter,this being the same portion of the sdeband as was passed through theiter 22.

The audio-frequency amplifier 23 is coupled to both signal channels toprovide a combined indication-of intelligence borneby the.. frequencyband il,

If each signal channel passed exactly half of the frequency band I I, itwould be impossib to eliminate an Yundesired carrier lying closelyadjacent the center of the band. For this reason t.ie filters 22 and 25are arranged to pass slightly less than half of the band as shown by thesloping response curves in Figs. 2C and 2D and as shown more clearly inFig. 3 which is a combination of Figs. 2C and 2D. The filters preferablyare arranged to have an attenuation of twenty decibels at the centerfrequency and to have an attenuation of sixty decibels at frequenciesslightly beyond the center. The s oping response curve of the filtersresults in attenuating a small band of frequencies (preferablyapproximately 200 cycles) near the center of the band. This arrangementwill at tenuate any undesired signal lying near the center of the bandand will also attenuate voice frequencies or other intelligencefrequencies lying between about 1900 cycles and 2100 cycles. Theattenuation of these voice frequences does not seriously distort thereceived signal.

The squelch circuits 23 and 2l, one of which is in each signal channel,are desirable to obtain a high signal to noise ratio. When the signaloutput of the lter in each channel falls below a predetermined usefullevel the squelch circuit cuts off the output. Without these squelchcircuits noise Would always come through each of the signal channelsdespite the fact that each channel is alternately operable in timeinsofar as the ole-v sired signal is concerned. With the squelchcircuit, when the upper .signal channel in Fig. 2 is receiving a..segment vin the group including the carrier 13 -for example., the lowersignal channel is completely cut off .and has no output; and similarly'when the lov/er signal channel is receiving a Vsegment in the group ofsegments including a carrier, the upper signal channel is completelycu-t oif. The squelch circuits provide the added advantage that shouldthe .amplitude .of the signal passing through the filters fall below auseful level, the squelch circuits will lcut off the output of thesignal channel. For example, during the time intervals when the group ofsegments including the carrier I2 is transmitted it is quite possiblethat no intelligence lying in the frequency band from 2,000 to 4,000cycles is being transmitted. In this :event only noise would comethrough the flower signal channel. However. the squelch circuit 'cuts onanyo-utput from the lower signal channel z underssuch conditions,thereby Jkc. and 999 kc. lillustrated occupies a total band of 6 kc.with increasing the signal-to-noise ratio over that of conventionalmodulation systems,

Figs. 5-9 illustrate a modified embodiment of the invention wherein aband of intelligence frequencies 4 kc. wide is received while utilizinga band only 1 kc. wide at any given instant.

A transmitter (not shown but similar in principle to the transmitter Iof Fig. 1) transmits a wave shown diagrammatically in Fig. 9. This wavecomprises a frequency band of waves bearing intelligence to becommunicated and a plurality of carrier waves provided in alternatingsequence respectively on opposite sides of the center of the band toprovide a plurality of groups of segments, the segments of each grouprecurring cyclically in timed sequence, there being as many groups asthere are different carriers, and each segment including intelligencebearing waves. In Fig. 9 the intelligence band is indicated at I I(being stippled) and comprises a band of frequencies 4 kc. in width atany given instant. One segment comprises a carrier |I2a and theintelligence bearing frequencies of the -band during the time thecarrier I I2a is on. Another segment of the same group includes acarrier H2b and intelligence frequencies associated therewith; a secondgroup of segments includes carrier segments I |3a, ||3b, etc. andassociated inteli'fence frequencies; a third group of segments includescarrier segments I64a, |6412, etc. and associated intelligencefrequencies; and a fourth group of segments includes carrier segments|6511, I55b, etc. and associated intelligence frequencies. The mean orcenter frequency is shown at H4 and an undesired signal (assumed to lieone half kc. above the center frequency) is Shown at I |5 The shadedportion I IIi of the band is that portion utilized at the receiver aswill r appeal'.

As illustrated the band is 4 kc. wide at any given instant, but a totalband width of 6 kc. is transmitted in order to space the carriers inalternating sequence at opposite sides of the center of the band. Forexample, the carrier I|2 may have a frequency of 999 kc. and haveassociated therewith a superior sideband of intelligence bearingfrequencies extending between 999 kc. and 1003 kc. Carrier ||3 mav havea frequencv of 1001 kc. (being located on the opposite sideI of thecenter of the band from the carrier II2) and have associated therewithan inferior sideband of intelligence bearing frequencies extendingbetween 1001 kc. and 997 kc.; the carrier |64 may have a frequency of997 kc. and have 4associated therewith a superior sideband ofintelligence bearingr frequencies extending between 997 kc. and 1001kc.: and the carrier |65 may` have .a frequency of 1003 kc. and haveassociated therewith an inferior sideband extending between 1003 Whilethe transmitted wave as 4 kc. within this band being transmitted at anyAgiven instant, if desired the total width of the transmitted band canbe cut down to one (1) kc. by a switch or frequency inverting device inthe transmitter so arranged that instead of transmitting a continuousspectrum of the entire band of intelligence frequencies (4 kc.) 4only a-portion (as for example one-fourth) of the intelligence bearing wavesmay be transmitted at any i given time and the frequency of the carrierand of the intel'igence or modulation waves may be simultaneouslyswitched to provide a continuous band of 1,000 cycles of constantfrequency limits- 1'. e. between 1,000 lso. and 1901 kc., the

10 carrier which shifts in frequency with each switching operation beingsuppressed.

Furthermore in the system of Fig. 1-4 and in the system Fig. 5-9, thecarrier may be switched alternately on opposite sides of the center ofthe band in the manner shown in the earlier mentioned publications by F.A. Polkinghorn and A. A. Oswald or if desired the carrier may beswitched by utilizing a conventional single sideband transmitter andinverting the carrier and the sideband of intelligence bearingfrequencies by means of a frequency inverting arrangement similar tothat including the A oscilator, the B oscillator and switch 3|. Ifeither of these switching methods are used the switching frequency mustbe either above or below the range of the intelligence bearing frequencyband. An alternative means of switching at the transmitter is by meansof voice or modulation control, as by rectifying the positive peaks ofthe voice frequenciesand using the rectified voltage as a controlvoltage for causing switching to one carrier position, and similarlyrectifying the negative peaks of the voice frequencies and using thisvoltage to control switching to another carrier position. The advantageof this type of control is that no carrier is on during voice luls.

Referring now to Fig. 5, a portion of the receiver is illustrated. InFig. 5 a mixer stage |30 has coupled to its input tlte movable element|3|a of a switch I3I. One stationary contact |3|b of this switch iscoupled to an oscillator designated A and the other stationary contact|3Ic `is coupled to an oscillator designated B', this arrangement beingsimilar to that shown in Fig. 2.

A desired signal of the character represented in Fig. 9 is also fed tothe input of the mixer, with the mixer being preceded by one or morestages of R. F. amplification, frequency conversion, and intermediatefrequency amplification.

In the receiver illustrated means are provided defining a. plurality ofsignal channels equal in number to the number of groups of segments ofthe transmitted wave, there being four such channels illustrated, anddesignated respectively as D, E, F, and G. The channel D includesgincascade arrangement a filter-amplifier |35, a detector |36, a low passaudio filter I3'I and a squelch circuit |38. Channel E includes afilteramplifier |39, a detector |40, a low intermediate pass audiofilter IM and a squelch circuit |42. Channel F includes afilter-amplifier |43, a detector |44, a high intermediate pass audiofilter |45 and a squelch circuit |46; and channel G includes afilter-ampier |41, a detector |48, a high pass audio filter |59 and asquelch circuit |50. Each of the filter-amplifiers leas its inputcoupled in parallel to the output of the mixer |30, and each of thesquelch circuits has its output coupled in parallel to the input of anaudio frequency amplifier |28 which in turn is coupled to a speaker |29.

Fig. 6 shows the frequency response or selectivity curves of each of thefilter-amplifiers. Amplifier in channel D has a frequency response suchthat it passes only frequencies lying within a band extending from apredetermined center frequency designated 0 in Fig. 6 to 1 kc. belowsaid center frequency, as shown by the curve |35a, in Fig. 6.Filter-amplifier |39 has a frequency response such that it passes a band2 kc. in width, one kc. lying on either side of-the predetermined centerfrequency as shown at |39a. Filter-amplifier |43 passes a band 3 kc. inwidth having its upper end at the predetermined fr equency as shown at|43a; and filter-amplifier |41 passes aband i kc. in` width extendingupwardly from a frequency 1 kc. below the predetermined center frequencyas shown at Ifile.

Fig. 7 is a combined View showing the operation of the filter-amplifiersin receiving a signal such as that shown in Fig. 9, the shaded portionbeing similar to the shaded portion in Fig. 9 and representing theportion of the transmitted band which is ultimately utilized to providean indication of intelligence which is transmitted. Assuming the centerfrequency of the mixer |30 to be 500 kc. and the center frequency of theconverted desiredv signal at the mixer output to be V100 kc. as shownin` Figs. 6 and 7, A oscillator may have a frequency of 600 kc. and Boscillator may have a frequency of 400 kc. Witnthe B- oscillatorconnected to the mixer as shown in Fig. carrier ||2 would be convertedfrom its intermediate frequency of 499 ko. at the input of the mixer toa frequency of 99 kc. as represented in Fig. '7 at ||2 the carrier H2heterodyning with the 400 cycle wave from the B oscillator tov provide awave of 99 kc. shownvin Fig. 7 as being 1 kc. below the predeterminedcenter frequency. Because of the selectivity of filter-amplifier onlythat portion of the band comprising frequencies from 0 toV 1 kc. abovethe carrier ||2 would be passed by the filter# amplifier |35, and theundersired signal would be converted in the mixer from a frequen'cy of500.5 kc.. to a converted frequency of 100.5 kc. represented in Fig. 7at H5' and lying outside the pass range of the iilter-amplier |35.

In Fig. 8 the frequency response curve of the low pass audio filter |31is shown at Isd, this filter passing audio frequencies in the range from0 to vabout 1,000 cycles. After demodulation in the detector stage |30the shaded portion H5 of the band passes through the filter, this shadedportion comprising audio frequencies from 0 to about 1,000 kc. Even ifthe undesired signal passedv through the filter-amplifier |35 it wouldheterodyne with the carrier H2' to produce a beat note of 1500 cyclesand would be eliminated. Garrier Yl I3 would be converted in the mixer|30 from its F. frequency of 501 kc. to a frequeny of 101 kc., which is1 kc. above the predetermined center frequency of 100 kc. This car? rierwould be passed through channel E as shown, the filter-amplifier |39passing the converted carrier H3', the converted undesired signal |15and a band of frequencies extending 2,000 cycles below the frequency ofthe carrier H3. However, after demodulation in the detector f4.0 onlythose frequencies spaced from 1,000 to 2,000 cycles from the carrier||3' will be passed by the low intermediate pass audio lter 14|, theresponse curve of which is shown at IM in Fig. 8. Since the undesiredsignal would heterodyne with the carrier ||3' to produce a beat noteof500 cycles, it would be eliminated by the lter |4|-.

Similarly carrier |64 and a portion of its family of sidebands arepassed through signal channel F, the filter-amplifier |43 in thischannel accepting thec'arrier converted to 97 kc. as shown at lland thatportion of the band i lying within 3,000- cycles on the high side of thecarrier. Thus, the nlteraainolier eliminates the undesired signalassociated withthis segment of the band, and, as shown in Fig. 8, thehigh intermediate pass audio filter |45 passes only those frequenciesspaced between 2,000 and 3,000 cycles from the carrier afterdemodulation. Consequently, even though the filter-amplifier |43 alsoaccepts car-i f5 rier I2 and the lower 1,000 cycles of its associatedside band, this segment is blocked by the audio filter |45. Noundesirable heterodyning occurs of course between the segments includingthe carrier ||2 and the segments including the carrier |04 because theyare received at diii'erent times. Those segments including carrier |05are passed through signal channel G. Carrier |025 is converted from itsI. F. frequency of 503 kc. to a frequency of 103 kc. designated at |65'in Fig. "i, and the entire segment of sidebands is passed by thefilter-amplifier |47. However, the undesired signal and all frequencieslying within 3,000 cycles of the carrier are eliminated by the high passaudio filter. The response curve for this audio filter is shown at |49'in Fig. 8, and the filter passes only frequencies lying between3,000-and 4,000. cycles. Upon detection, the undesired signalheterodynes with the carrier to create a beat note of 2,500 cycles whichis blocked by the filter.

As shown in Fig. 8 the filters attenuate frequencies near the edge ofthe pass band of the filter so that undesired signals which are closelyadjacent the edge of the pass band are attenuated in a manner similar tothat described in connection with Fig. 3. Similarly the filterampliershave sloping response curves which attenuate signals near the edge oftheir pass band farthest from the carrier received by each amplifier,thus effectively eliminating undesired signals very close to the center.frequency and preventing unwanted portions of the desired carrier frompassing through. For example, referrn: to Fig. 7, the undesired signalH5 is within the band pass' of signal chamber E. If the carrier ||2'were also within the band pass of this signal channel the undesiredsignal H5' would heterodyne with the desired signal carrier H2 in thedetector |40 in the signal channel E to provide a beat note of 1500cycles, and this beat note would pass through the audio filter |4|. Thisundesirable action is prevented by providing a sloping response curvefor each of the lterampliers, and in the example given, as may be seenin Fier.Y 7, the carrier ||2 will be attenuated by the filter-amplifier|39 and will be effectively suppressed so that no undesired beat note isprovided within the band pass of the filter |4|. i

The squelch circuits operate in the same manner as described inconnection with Fig. 2, and the output of all the channels are connectedto the aiidio-frnnimncy amplifier |28 to provide a combined indication.

In the event an undesired signal lies on the other side of the centerfrequency, switching to the A oscillator will cause selection of adifferent portion of the band l and will eliminate the undesired signalin tbe manner shown in my earlier patent and in the application abovereferred to, In this event of course, the portion ||5 of the band whichis shadedin Figsf? and 9 will no longer be used, but another portionwill be selected to provide a combined indication.

While I have shown and described certain embodiments of my invention, itis to be understood that it is capable of many modifications. Changes,therefore, in the construction and arrangement may be made withoutdeparting from the spirit and scope of the invention as disclosed in theappended claims.

I claim:

1. Communication apparatus of the character described ,for eliminatingan undesired signal closely adjacent a desired signal, including: meansfor providing a desired signal comprising a frequency band of wavesbearing intelligence to be communicated and a plurality of carrier Wavesprovided in alternating sequence respectively on opposite sides of thecenter of said band to provide a plurality of groups of segments, thesegments of each group recurring cyclically in timed sequence, and eachsegment including intelligence bearing waves; receiving means includinga plurality of signal channels each passing a diiierent group ofsegments and frequencies comprising only a portion of the intelligencebearing waves in such segments; and means coupled to the output of allof said channels for providing a combined indication of intelligenceborne by said portions of the band.

2. Communication apparatus of the character described for eliminating anundesired signal closely adjacent a desired signal, including: means forproviding a desired signal comprising a frequency band of waves bearingintelligence to be communicated and a plurality of carrier wavesprovided in alternating sequence respectively on opposite sides of thecenter of said band to provide a plurality of groups of segments, thesegments of each group recurring cyclically in timed sequence, therebeing as many groups as there are carriers and each segment includingintelligence bearing waves; receiving means including a plurality ofsignal channels each passing a diiferent group of segments and adiiTerent frequency portion of the intelligence bearing waves in suchsegments; and means coupled to the output of all of said channels forproviding a combined indication of intelligence borne by said portionsof the band.

3. Apparatus of the character claimed in claim 2, wherein said receivingmeans include a plurality of lters, each passing frequencies comprisinga portion of the intelligence bearing waves in such segments differentfrom, but closely adjacent to, the intelligence bearing waves passed byanother of said filters, so that substantially entirely al1 frequenciesof waves bearing intelligence are passed by said signal channels.

4. Apparatus of the character claimed in claim 2, wherein each signalchannel includes means for demodulating a portion of the desired signal.

5. Apparatus of the character claimed in claim 2, wherein each signalchannel includes electronic means for cutting off its output during thetime it is not passing a group of segments and a portion of theintelligence bearing waves in such segments.

6. Apparatus of the characterclaimed in claim 2, wherein the receivingmeans includes apparatus for changing the frequency of said segments tochange the portion of said band passed by each respective signalchannel, together with switch means for selecting a desired portion,undesired signals in the non-selected portion being eliminated.

7. Communication apparatus of the character described for eliminating anundesired signal closely adjacent a desired signal, including: means forproviding a desired signa1 comprising a frequency band of waves bearingintelligence to be communicated and first and second carrier wavesprovided in alternating sequence respectively adjacent the upper andlower edges of said band to provide first and second groups of segments,the segments of each group recurring alternately and cyclically in timedsequence, and each segment including a carrier and intelligence Il (lbearing waves; receiving means including a rst signal channel passingonly one of said groups of segments and frequencies comprising only aportion of said intelligence bearing waves in such segments, and asecond signal channel passing only the other of said groups of segmentsand frequencies comprising only a portion of said intelligence bearingwaves in such segments; and means coupled to the output of all of saidchannels for providing a combined indication of intelligence borne bysaid portions of the band,

8. Apparatus of the character claimed in claim 7, wherein said iirstsignal channel passes frequencies comprising substantially half of saidintelligence bearing waves, and said second signal channel passesfrequenciescomprising substantially the other half of said intelligencebearing waves.

9. Apparatus of the character claimed in claim 7, wherein said firstsignal channel passes frequencies comprising less than half of saidintelligence bearing Waves and said second signal channel passesfrequencies comprising less than the other half of said intelligencebearing Waves, undesired signals lying adjacent the center frequency oi'said band being eliminated.

10. Apparatus of the character claimed in claim 7, wherein the receivingmeans includes apparatus for inverting the frequency of said segments toinvert the portion of said band passed by each respective signalchannel, together with switch means for selecting a desired portion,undesired signals in the non-selected portion being eliminated.

11. Communication apparatus of the character described for eliminatingan undesired signal closely adjacent a desired signal, including: meansfor providing a, desired signal comprising a frequency band of wavesbearing intelligence to be communicated and first and second carrierwaves provided in alternating sequence respectively adjacent the upperand lower edges of said band to provide further and second groups ofsegments, the segments of each group recurring alternately andcyclically in timed sequence,

and each segment including a carrier and intel- Vligence bearing Waves;means defining a rst signal channel, including a rst lter-ampliiierhaving a selectivity such that it receives at least a portion of one ofsaid groups of segment-s, means coupled to said filter-amplifier fordemodulating the received carrier and a iii-st iilter coupled to thedemodulating means, said lter passing frequencies comprisingsubstantially half of said intelligence bearing waves; means dening asecond signal channel, including a second lter-amplier having aselectivity such that it receives vat least a portion of the other ofsaid groups of segments, means coupled to said second filterarnplier fordemodulating the received carrier and a second filter coupled to thelast mentioned demodulating means, said second iilter passingfrequencies comprising substantially the other half` of saidintelligence bearing waves; and means coupled to the output of both ofsaid signal channels for providing a combined indication of intelligenceborne by said band.

l2. Apparatus of the character claimed in claim ll, wherein theselectivity of each lter-amplifier is such that each lter-ampliiiersuppresses the carrier of the segment not received, and wherein themeans for providing a combined indication comprises an -audio-frequencyamplifier coupled to the output of both of said iilters and having apass band covering the band of intelfligence bearing waves passed byboth of said 'filters 13. Communication apparatus of the characterdescribed for eliminating an undesired signal closely adjacent a desiredsignal, including: means for providing a desired signal comprising afrequency band of Waves bearing intelligence to be communicated andiirst and second carrier Waves provided in alternating sequencerespectively adjacent the upper and lower edges of said band to providefirst and second groups of segments, the segments of each grouprecurring alternately and cyclically in timed sequence, and each segmentincluding a carrier and intelligence bearing waves; means for providingtwo separate heterodyning waves, one having a frequency equal to' themeans frequency of the desired signal plus a certain frequency and theother having a frequency equal to the mean frequency of the desiredsignal less said certain frequency whereby heterodyning of saidheterodyning waves With the desired signal changes the frequency of thedesired -signal to the same frequency in both cases While changing thatof the undesired signal to different frequencies; means for selectingone of said heterodyning waves to heterodyne with the desired signal;means definingV a first signal channel, including a iirstfilter-amplifier having a selectivity such that it receives at least aportion of one of said groups of segments, means coupled to saidfilter-ampli- `Iier for demodulating the received carriel` and a firstiilter coupled to the demodulating means,

said filter passing frequencies comprising substantially half of saidintelligence bearing wave; means defining a second signal channel,including a second lter-ampliiier having a selectivity such that itreceives at least a portion of the other of said groups of segments.means coupled to said second iilter-ampliiier for demodulating thereceived carrier and a second iilter coupled to the last mentioneddemodulating means, said second filter passing frequencies comprisingsubstantially the other half of said intelligence bearing waves; andmeans coupled to the output of both of said signal channels forproviding a combined indication of intelligence borne by said band.

14. Communication apparatus of the character described for eliminatingan undesired signal closely adjacent a desired signal, including: meansfor providing a desired signal comprising a frequency band of wavesbearing intelligence to be communicated and iirst and second carrierWaves provided in alternating sequence respectively adjacent the upperand lower edges of said band to provide first and second groups ofsegments, the segments of each group recurring alternately andcyclically in timed sequence, and each segment including a carrier andintelligence bearing waves; means deiining a iirst signal channel,including a. first filter-amplifier having a selectivity such that itreceives at least a portion of one of said groups of segments andsuppresses the carrier of the other group, means coupled to saidiiltor-amplier for demodulating the received carrier and a first iiltercoupled to the demodulating means, said filter passing frequenciescomprising substantially half of said intelligence bearing waves; meansdefining a second ,signal channel, including a second lter-amplifierhaving a selectivity such that it receives at least a portion of theother of said groups of segments, and suppresses the carrier of thegroupnot received, means coupled to said secondA iilter-am'pliiier fordemodulating the received carrier and a second filter coupled to thelast mentioned demodulating means, said second filter passingfrequencies comprising substantially the other half of said intelligencebearing waves; means coupled to the respective filter in eachsignalchannel for cutting onf the output of the respective channel during thetime the channel is not passing a group of segments and a portion ofintelligence bearing waves and when the output of intelligence bearingwaves is below a predetermined level; and means including anaudio-frequency amplifier coupled to the output of both of said channelsfor providing a combined indication of intelligence borne by said band.

15. Communication apparatus of the character Y described for eliminatingan undesired signal closely adiacent a desired signal, including: meansfor providing a desired signal comprising a frequency band of Wavesbearing intelligence to be communicated first angl second carrier wavesprovided in alternatng sequence respectively adjacent the upper andlower edges of said band to provide and second groups of segments, theseunents of each group recurring alternately and cyc ically in timedsequence, and each segment incuding a carrier and intelligence bearingwave-s; means for provid'ng two separate heterodyning waves, one havinga frequency equal to the mean frequency of the desiredl signal plus acertain frequency and the other having a frequency equal to the meanfrequency of the desired signal less said certain frequency, wherebyheterodyning of said heterodyning waves with the desred signal changesthe frequency of the desired sijnal to the same frequency in both caseswhile changing that of the undesired signal to different freqfeneies;means for selectinf; one of said hetercdyning waves to heterodyne withthe desired signal; means definng a iirst signal channe, including afirst filter-amplifier having a selectivity such that it receives atleast a portion of one of said groeps of segments, and suppresses thecarrier of the other group, means coupled to iilter-ampl'iier fordemodulating the received carrier and a rirst iilter coupled to thedemodulating means, said filter passing frequencies conA icingsubstantially half of said intelligence bearing waves; means definingr asecond signal channel, ncuding a second filteramp'ifier having aselectivity such that it receives at least a portion of the other ofsaid groups of segments, and suppresses the carrier of the group notreceived, means coupled to said second filter-amplifier for demodulatingthe received carrier and a second filter coupled to the last mentoneddemodulating means, said second filter passing frequencies comprisingsubstantially the other half of said intel igence bearing Waves; meansco'fpied to the respective filter in each signal channei for cutting offthe output of the respective channel during the time the channel is notpassing a group of segments and a portion of intelligence bearing wavesand when the output of intelligence bearing waves is below apredetermined level; and means including an audio-frequency amp iiiercoupled to the output of both of said channels for providing a combinedindication of intelligence borne by said band.

16. Communication apparatus of the character described for eliminatingan undesired signal closely adjacent a desired signal, including: meansfor providing a desired signal comprising a frequency band of Wavesbearing intelligence to be communicated and a plurality of carrier Wavesprovided in alternating sequence respectively on opposite sides of thecenter of said band to provide a plurality of groups of segments, thesegments of each group recurring cyclically in timed sequence, therebeing as many groups as there are dierent carriers, and each segmentincluding a carrier and intelligence bearing waves; means defining apluraiity of signal channels equal in number to the number of saidgroups of segments, each channel including a filter-amplifier having aselectivity such that it receives at least a portion of a different oneof said groups of segments, means in each channel coupled to saidfilter-amplifier for demodulating the received carrier and a filter ineach channel coupled to the demodulating means, said filter passingfrequencies comprising only a portion cf said intelligence bearingwaves, the portion of said waves passed by the filter in each channelcomprising a fraction of the total intelligence bearing frequenciesdetermined by dividing said total frequencies by the number oi'channels, and each lter passing frequencies different from, but closelyadjacent to the frequencies passed by another of said filter, so thatsubstantially en,- tirely all frequencies of waves bearing intelligenceare passed by said signal channels; and means coupled to the output ofall of said channels for providing a combined indication of intelligenceborne by said portions of the band.

17. Communication apparatus of the character described for eliminatingan undesired signal closely adjacent a desired signal, including: meansfor providing a desired signal comprising a frequency band of wavesbearing intelligence to be communicated and a plurality of carrier wavesprovided in alternating sequence respectively on opposite sides of thecenter of said band to provide a plurality of groups of segments, thesegments of each group recurring cyclically in timed sequence, therebeing as many groups as there are different carriers, and each segmentincluding a carrier and intelligence bearing waves; means for providingtwo separate heterodyning Waves, one having a frequency equal to themean frequency of the desired signal plus a certain frequency and theother having a frequency equal to the mean frequency of the desiredsignal less said certain frequency, whereby heterodyning of saidheterodyning waves with the desired signal changes the frequency of thedesired signal to the same frequency in both cases while changing thatof the undesired signal to different frequencies; means for selectingone of said heterodyning Waves to heterodyne with the desired signal;means defining a plurality of signal channels equal in number to thenumber of said groups of segments, each channel including afilteramplifier having a selectivity such that it receives at least aportion of a different one of said groups of segments, means in eachchannel coupled to said filter-amplifier for demodulating the receivedcarrier and a filter in each channel coupled to the demodulating means,said filter passing frequencies comprising only a portion of saidintelligence bearing waves, the portion of said waves passed by thefilter in each channel comprising a fraction of the total intelligencebearing frequencies determined by dividing said total frequencies by thenumber of channels, and each filter passing frequencies diferent from,but closely adjacent to the frequencies passed by another of saidfilters, so that substantially entirely all frequencies of waves bearingintelligence are passed by said signal channels; and means coupled tothe output of all of said channels for providing a combined indicationof intelligence borne by said portions of the band.

18. Communication apparatus of the character described for eliminatingan undesired signal closely adjacent a desired signal, including: meansfor providing a desired sign-a1 comprising a frequency band of wavesbearing intelligence to be communicated and a plurality of carrier wavesprovided in alternating sequence respectively on opposite sides of thecenter of said band to provide a plurality of groups of segments, thesegments of each group recurring cyclically in timed sequence, therebeing as many groups as there are different carriers, and each segmentincluding a carrier and intelligence bearing waves; means for providingtwo separate heterodyning waves, one having a frequency equal to themean frequency of the desired signal plus a certain frequency and theother having a frequency equal to the mean frequency of the desiredsignal less said certain frequency, whereby heterodyning of saidheterodyning waves with the desired signal changes the frequency of thedesired signal to the same frequency in both cases While changing thatof the undesired signal to different frequencies; means for selectingone of said heterodyning waves to heterodyne with the desired signal;means defining a plurality of signal channels equal in number to thenumber of said groups of segments, each channel including afilter-amplifier having a selectivity such that it receives at least aportion of a different one of said groups of segments, means in eachchannel coupled to said lter-amplier for demodulating the receivedcarrier and a lter in each channel coupled to the demodulating means,said filter passing frequencies comprising only a portion of saidintelligence bearing waves, the portion of said waves passed by the lterin each channel comprising a fraction of the total intelligence bearingfrequencies determined by dividing said total frequencies by the numberof channels, and each filter passing frequencies different from, butclosely adjacent to the frequencies passed by another of Said filters,so that substantially entirely all frequencies of waves bearingintelligence are passed by Said signal channels; means coupled to therespective filter in each signal channel for cutting oi the output ofthe respective channel during the time the channe1 is not passing agroup of segments and a portion of intelligence bearing waves and whenthe output of intelligence bearing waves is below a predetermined level;and means including an .audio-frequency amplifier coupled to the outputof both of said channels for providing a combined indication ofintelligence borne by said band.

JAMES L. A. MCLAUGHLIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,816,953 Bown Aug. 4, 19312,179,106 Taylor Nov. 7, 1939 2,364,863 McLaughlin Dec. 12, 19442,393,224 Van Dissel Jan. 15, 1946 2,407,260 Dickieson Sept. 10, 19462,411,206 Guanella Nov. 19, 1946 2,480,870 McLaughlin Sept. 6, 1949

